There could be a risk involved, if:
1) The image is uploaded inside the Document Root
2) Have some malicious code inside (ex: a php shell) that is not validated
3) The Web Server somehow executes this malicious code (for example, you can put php code inside a
GIF, after the magic number, and the web app include/require this file in a php script, then the php
engine will execute the php code when it sees the php opening tag, even if it's inside the image)

Also, beware of the null byte, or example, can you upload a filename like this phpShell.php%00.jpg?

The best practice is:
a) Always upload outside the DocRoot
b) Do not trust Content-type and filename headers nor the magic number
c) Validate the content of the file
d) Scan the file with an antivirus
e) Be careful if you include/require the file

Hope it helps!

Best regards.

On 02/12/14 15:44, Jyotiranjan Acharya wrote:
> If you are able to upload a file with a changed extension, then will
> that be a problem?
> For example, you can not ,in any way, upload a .exe or .php/.jsp/.asp
> file directly into a web App, but you can by changing their extension
> to .JPG. What is the risk in such a case?
>
>
>
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